An MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr or Co/Cu multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure comprising two essentially uncoupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is "pinned". The pinning may be achieved by depositing the ferromagnetic layer to be pinned onto an antiferromagnetic iron-manganese (Fe--Mn) layer so that these two adjacent layers are exchange coupled. The unpinned or "free" ferromagnetic layer may also have the magnetization of its extensions (those portions of the free layer on either side of the central active sensing region) also fixed, but in a direction perpendicular to the magnetization of the pinned layer so that only the magnetization of the free-layer central active region is free to rotate in the presence of an external field. The magnetization in the free-layer extensions may also be fixed by exchange coupling to an antiferromagnetic layer. However, the antiferromagnetic material used for this must be different from the Fe--Mn antiferromagnetic material used to pin the pinned layer. The resulting structure is a spin valve (SV) magnetoresistive sensor in which only the free ferromagnetic layer is free to rotate in the presence of an external magnetic field. U.S. Pat. No. 5,159,513, assigned to IBM, discloses a SV sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer. U.S. Pat. No. 5,206,590, also assigned to IBM, discloses a basic SV sensor wherein the free layer is a continuous film having a central active region and end regions. The end regions of the free layer are exchange biased by exchange coupling to one type of antiferromagnetic material, and the pinned layer is pinned by exchange coupling to a different type of antiferromagnetic material. The SV sensor described in the '590 patent has the disadvantage that it is difficult to fabricate. If subtractive processing techniques are used, it is difficult to precisely remove the top layers of the sensor down to the free layer without damaging or thinning the free layer. Also, the free layer may be a combination of several layers, including very thin layers only several angstroms in thickness, and the top layers of the free-layer structure may be damaged during the removal process. If additive processing is used, it is no longer possible to deposit the entire spin valve layers in one vacuum cycle so that the integrity of the interface between the free layer and the spacer layer becomes compromised.
The copending '714 application describes an AMR sensor wherein the MR element has discontinuous end regions abutting the central sensing region with the end regions being longitudinally biased by antiferromagnetic layers.
What is needed is a SV sensor having a free ferromagnetic layer with improved end regions for coupling to the antiferromagnetic layer.